This invention relates to novel apparatus for measuring heights of peaks above an arbitrary level by statistical means not limited to roughness or minimum area, and more particularly to a synthetic-aperture radar system for measuring the average height of terrain peaks or ocean waves.
Much work has been done on the remote measurement of ocean wave heights using nonimaging radar, and a review of such techniques is presented by D. E. Barrick, Remote Sensing of Sea State by Radar, Remote Sensing of the Troposphere, edited by V. E. Derr, Ch. 12, U.S. Government Printing Office, Washington, D.C., 1972. G. T. Ruck, D. E. Barrick, and T. Kaliszewski, Bistatic Radar Sea State Monitoring, Battelle Columbus Laboratories, Columbus, Ohio, Rep. 1388-19, 1971, have analyzed the possibility of using two-frequency radar interferometry for measuring wave slopes, and Weissman, et al. have demonstrated the operation of a radar system utilizing this technique both in laboratory wave tank measurements and flights over the Caribbean, [D. E. Weissman, Two Frequency Radar Interferometry Applied to the Measurement of Ocean Wave Height, IEEE Trans. on Ant. and Prop., AP-21, 649=656, 1973; D. E. Weissman, C. T. Swift, W. L. Jones, Jr., J. W. Johnson, W. L. Grantham, J. A. Howell, J. C. Fedors, and J. J. Davis, A Dual Frequency Radar for Ocean Roughness Sampling, URSI Fall Meeting, Boulder, Colorado, 1974.] The possibility of measuring ocean wave heights by studying the return from a short-pulse radar has also been demonstrated by B. D. Zamarayev and A. I. Kalmykov, On the Possibility of Determining the Spatial Structure of an Agitated Ocean Wave by Means of Radar, Izvestia, USSR Academy of Sciences -- Atmospheric and Ocean Physics, 5, 64-66, 1969, and by D. L. Hammone, R. A. Manella and E. J. Walsh, Short Pulse Radar Used to Measure Significant Wave Height and Surface Wind Speed, URSI Fall Meeting, Boulder, Colorado, 1975.
The principal concept involved in the wave height measurement, which is common to the interferometric techniques utilizing radar sensors, is that the resultant intensity for the electric field scattered from the ocean wave at some arbitrary point in space changes as the wavelength or angle of illumination for the electromagnetic radiation varies. This variation is dependent on the height difference between the crest and trough of the wave, and a measurement of the rate of this variation provides an indication of the ocean wave height. This principle has been used before in the design and operation of the tellurometer for the precise determination of range differences, [T. L. Wadley, Electronic Principles of the Tellurometer, Trans. South African Inst. Electr. Engrs., 49, 143-161, 1958.] The various radar techniques for measuring ocean wave heights differ in the handling of the radar data, which is governed by the constraints set by the particular radar instrument being used, the accuracy and spatial resolution of the measurement being related to the bandwidth and the resolution of the radar system involved.
An extensive theoretical and experimental treatment of the utilization of this concept to measure the height of surface features within a resolution cell in images obtained by a tunable dye laser has been performed by Jain, the present inventor [A. Jain, A Wavelength Diversity Technique for Smoothing of Speckle, Ph.D. Thesis, California Institute of Technology, Pasadena, California, 1975], [N. George and A. Jain, "Space and Wavelength Dependence of Speckle Intensity," Appl. Phys., 4, 201-212, 1974], and [George, et al., "Experiments on the Space and Wavelength Dependence of Speckle," Appl. Phys., 1, 157, 1975]. The surface heights obtained by varying the wavelength can be determined quite accurately to the order of a wavelength, the accuracy improving with greater heights.
While the wavelengths of the ocean waves can be determined by inspection of the radar imagery or by obtaining its Fourier transform [W. E. Brown, Jr., C. Elachi, and T. W. Thompson, "Radar Imaging of Ocean Surface Patterns," J. Geophys. Res., Vol. 81, No. 15, pp. 2657 (May 1976); C. Elachi, "Two Dimensional Wavelength Spectrum of Swells Across the North Atlantic," J. Geophys. Res., (in press)], it is also of considerable interest to measure the heights of the ocean waves. A method of processing the images obtained by synthetic aperture radar to determine the heights of ocean waves at localized areas of interest in the images, and experimental results demonstrating this technique are presented.